Circuit breakers having push-pull actuators are typically used in aircraft electrical systems. Due to the limited space and weight available on aircraft, the circuit breakers used in aircraft electrical systems should provide both a manual switching device for turning equipment on and off in order to obviate the need for separate switches for each piece of equipment as well as overcurrent protection. Accordingly, circuit breakers having a short stroke push-pull actuator that terminates in an annular flange are preferred in aircraft applications since the operator can easily close the circuit breaker into an “equipment on” position by a push-button action or open the breaker into an “equipment off” position by pulling upwardly on the annular flange on the end of the actuator. Such push-pull actuated circuit breakers can also be made compactly, which allows them to be densely arranged in the limited space provided by aircraft electrical control panels, which often must accommodate hundreds of circuit breakers.
During certain aircraft maintenance procedures, it is essential for the safety of the maintenance workers that the push-pull actuators of some of the circuit breakers remain in a pulled-out, “equipment off” position. Consequently, the prior art circuit breaker locking device 1 illustrated in FIGS. 1A and 1B was developed. This locking device 1 is designed to lock a circuit breaker 3 having a push-pull actuator 5 that is slidably mounted in a circuit board 6. The push-pull actuator 5 includes a shaft section 7 that terminates in annular flange 8. The actuator 5 is reciprocally movable with respect to the circuit board 6 and may be may be pulled up into the “circuit open/equipment off” position illustrated in FIG. 1A, or pushed down into the “circuit closed/equipment on” position illustrated in phantom in FIG. 1A.
The prior art locking device 1 is formed from a pair of mirror-symmetrical body sections 10a, 10b, each of which has a semi-cylindrical outer surface 12 and a flat inner surface 14. A semi-annular groove 16 circumscribes the semi-cylindrical outer surfaces 12 of both of the body sections 10a, 10b near the proximal ends of these components. These semi-annular grooves 16 receive a joining member 18 in the form of an elastic O-ring that pulls the two body sections 10a, 10b together in the position illustrated in FIG. 1B. As is best seen in FIG. 1A, both body sections 10a, 10b include proximal and distal semi-cylindrical recesses 20, 22 that are complementary in shape to the shaft section 7 and annular flange 8 of the push-pull actuator 5, respectively. The semi-cylindrical recesses 20, 22 connect at interface 23 shown in FIG. 1A. Each of the body sections 10a, 10b includes, at its distal end, a lever member 24 formed as shown from a 30° cut-out section. The lever members 24 of the opposing body sections 10a, 10b converge at pivot line 26 as shown. Finally, body section 10a includes, at the upper part of its flat inner surface 14, a protrusion 27a and a cavity 27b which interfit with a corresponding, complementary-shaped cavity 27b and protrusion 27a of the other body section 10b. The interfitting protrusions 27a and cavities 27b prevent the two body sections from axially sliding out of alignment when they are joined by the elastic joining member 18 in the position illustrated in FIG. 1B.
In operation, the user grasps and pulls together the lever members 24 of the opposing body sections 10a, 10b with sufficient force to overcome the elastic force applied by the resilient joining member 18. Consequently, the opposing body sections pivot apart along line 26. The resulting 60° spread of the body sections 10a, 10b allows the device 1 to receive the push-pull actuator 5 of the circuit breaker 3. When the user releases the lever members 24, the resilient joining member 18 pulls the body section 10a, 10b back together into the position illustrated in FIG. 1B such that the shaft section 7 and the annular flange 8 of the actuator 5 are respectively captured by the proximal and distal semi-cylindrical recesses 20, 22. In the past, both the diameter D1 of the shaft sections 7 and the diameter D2 and axial length L1 of the annular flanges 8 were of standard and uniform size. Accordingly, the proximal and distal recesses 20, 22 were sized to be complementary in shape to the standard-sized shaft sections 7 and annular flanges 8 and to have corresponding diameters and D1′, D2′ and a length L1′ that were only slightly larger than diameters and D1, D2 and length L1. This prior art locking device 1 achieves a locking action by means of mechanical interference between the bottom surface of the annular flange 8 and the interface 23 between the proximal and distal semi-cylindrical recesses 20, 22. Such mechanical interference effectively immobilizes the actuator 5 from any axial movement and effectively locks the actuator 5 in the “circuit open/equipment off” position illustrated in FIG. 1A.
While the prior art locking device 1 works well to lock any one of a set of circuit breakers 1 having push-pull actuators 5 in which the shaft sections 7 and annular flanges 8 are all of a same size, problems arise when the radii of the shaft sections 7 and annular flanges vary. These problems have been exacerbated recently with the availability of button-like plastic collars that may be snap-fitted over the original flanges. These button-like plastic collars are available in a variety of colors, and the applicant has observed that some aircraft maintenance crews are attaching them over the original annular flanges of the circuit breakers in order to indicate, by color coding, the particular electrical system or component that the circuit breaker controls. Such plastic collars also advantageously facilitate the grasping and pulling out of the actuator into the “circuit open/equipment off” position. However, because such collars also have the effect of increasing both the radius and the thickness of the annular flange along its axis, the prior art locking device 1 may not operate to reliably lock the actuator 5 in the pulled-out, “circuit open/equipment off” position illustrated in FIG. 1A. The problems created by such plastic collars have been made worse due to the fact that they come in a variety of sizes. In cases where the largest collars are applied over the flanges 8, it may not possible to spread the body sections 10a, 10b far enough apart with the lever members 24 to allow the user to receive the flange 8 of the actuator 5 at all. And in cases where the body sections 10a, 10b can be spread far enough apart to receive the collar-enlarged flange 8, the edges of the proximal recesses 20 of the body sections 10a, 10b will simply clamp on to opposing sides of the flange 8 with the body sections 10a, 10b in a partially spread position such that a pushing force on the device may depress the actuator into a current-conducting “equipment on” position, creating a potentially unsafe condition.